1. Field of the Invention
The present invention relates to an ultrasonic probe made by arranging in two dimensions a plurality of electroacoustic transducing elements and to a diagnostic ultrasound system that uses the transducing elements for imaging. More particularly, this invention is concerned with a diagnostic ultrasound system capable of electronically controlling the beam width of an ultrasonic beam even in a slice direction orthogonal to a scan direction.
2. Description of the Related Art
The modality of diagnostic ultrasound, in which ultrasonic pulses are irradiated into a body and waves reflected from each tissue are used to acquire biomedical information, has the advantage of no possibility of causing an exposure-related injury unlike X-rays and a possibility of diagnosing a soft tissue without the use of a contrast medium.
In an ultrasonic probe used for a currently most prevailing system of an electronic scanning type, a plurality of electroacoustic transducing elements (hereinafter transducers) are generally arranged one-dimensionally, and controlled and driven mutually independently in order to change a transmission or reception direction of ultrasonic waves at a high speed and thus enable real-time production of tomographic images of a living body.
FIG. 1 shows a structure of an ultrasonic probe having transducers arranged one-dimensionally. In other words, the ultrasonic probe has a plurality of transducers 1 arranged one-dimensionally in a direction in which ultrasonic waves are scanned.
As shown in FIG. 1, each transducer 1 has an electrode 29 on a medium side thereof (living body side) through which an ultrasonic wave is transmitted or received and on an opposite side thereof (system side). The electrodes 29 on the system side are connected to signal lines 30, while the electrodes 29 on the living body side have the front sides thereof connected to an impedance matching layer 31. The impedance matching layer 31 adjusts a difference (a product of a density and sound velocity) in acoustic impedance between the living body and transducers 1, and enables ultrasonic pulses having a few waves to enter the living body efficiently.
Furthermore, an acoustic lens 32 made of a silicon rubber or the like is affixed to the impedance matching layer 31. The acoustic lens 32 fills the role of focusing an ultrasonic beam at a given distance in a slice direction orthogonal to a scan direction. Moreover, a back load member (backing) 33 is affixed to the electrodes 29 on the system side. The back load member 33 is designed to absorb unwanted ultrasonic waves irradiated in a direction opposite to a living body, and also fills the role of a support for the transducers 1.
Next, the principles of an (linear) electronic scanning type diagnostic ultrasound system having a thus-configured ultrasonic probe are shown in FIG. 2. Electronic switches 2 mounted on a main unit of the system are connected to the M transducers 1, which are arranged one-dimensionally in the probe, by way of signal lines. In the main unit of the system, the electronic switches 2 are connected to a transmitter 4 and receiver 5. Moreover, an output of the receiver 5 is displayed as an ultrasonic tomographic image on a TV monitor 6. In the linear electronic scanning type system, during one cycle of receiving ultrasonic wave, m adjoining transducers 1 of the M transducers 1 are driven simultaneously. The m transducers are shifted one by one at every transmission or reception.
Talking of each transmission wave or received wave, first, a reference pulse output from the transmitter 4 is sent to a delay circuit 3 that introduces a delay time necessary to focus an ultrasonic beam. Focusing an ultrasonic beam is intended to improve the resolution of an ultrasonic image. Transmission pulses lagging by different delay times are applied as driving signals to the m adjoining transducers 1 selected by the electronic switches 2. The m transducers 1 then irradiate ultrasonic waves to a medium. By contrast, reflected signals of ultrasonic waves emanating from a living tissue are received by the m transducers 1.
Received signals are then sent to the delay circuit 3 via the electronic switches 2, delayed by substantially the same delay times as the previous ones, and then synthesized. The resultant signals are subjected to amplitude compression or envelope detection within the receiver 5. After being converted from an analog form to digital form, the signals are stored temporarily in an image memory. One image produced by repeating this kind of operation while shifting m transducers one by one is converted according to a standard TV format and displayed on the TV monitor 6.
When an ultrasonic probe having the aforesaid transducers arranged one-dimensionally is employed, it is possible to focus an ultrasonic beam in an arrangement (scan) direction of transducers by controlling delay times electronically; that is, using a delay circuit. A movement of a focal point in an irradiation direction (depth direction of a living body) can be made rapidly. In other words, it is possible to scan an ultrasonic beam with its small width retained in the depth direction owing to fast control of delay times.
By contrast, as far as the slice direction is concerned, as already mentioned, since beam focusing using an acoustic lens is adopted, the radius of curvature of the acoustic lens is fixed, and a focal point is also fixed at one position. It is therefore impossible to keep a beam width small in a wide range in the depth direction. A two-dimensional array probe is available as a means for solving this problem.
Now, a transducer driving method to be adopted when the number (n) of transducers lying in the slice direction is 8 will be described. FIG. 3 shows the basic driving method. As already described, for focusing an ultrasonic beam, signals sent from transducers of a group of transducers which are located at symmetric positions with respect to the center are delayed by the same delay time.
Any problem therefore does not occur even when the transducers 1 are interconnected as illustrated in advance. This method halves the scale of subsequent electronic circuits. In a probe, the transducers I and preamplifiers 10 are connected unitedly. The two-dimensional array of transducers 1 needs a smaller area per element than conventional transducers. This leads to a high impedance of an element, whereby it becomes hard to attain a sufficient signal-to-noise ratio.
It is therefore preferable to place the preamplifiers 10 closely to the transducers 1. That is to say, the transducers 1 and preamplifiers 10 are incorporated in an ultrasonic probe, and then connected to a receiving circuit in the main unit of a system by way of cables with the impedances of the transducers reduced sufficiently. On a transmitting side, driving pulses used to drive the transducers I are sent from the main unit by way of the same cables. However, the driving pulses bypass the preamplifiers 10, and then are sent to the transducers in order to drive the transducers.
FIG. 3 also shows in detail a single buffer preamplifier AMP. The preamplifier AMP has high-voltage protecting circuits 11-A and 11-B on its input and output stages thereof. A diode circuit 9 is placed in parallel to these circuits. Namely, impulses each having a peak value ranging from 100 to 200 volts are sent from the main unit to the probe by way of the cables during transmission. The driving pulses are sent to the transducers 1 via the diode circuits in order to drive the transducers for generation of ultrasonic waves. At this time, the preamplifiers 10 can avoid a high-voltage breakdown owing to the protecting circuits 11A and protecting circuits 11B each of which includes a diode and the like.
By contrast, during reception, received signals received by the transducers are input to the preamplifiers 10 via the protecting circuits 11-A. After amplified by a given quantity, the signals are output against a low output impedance onto the cables via the protecting circuits 11-B, and then sent to the receiving circuit in the main unit. At this time, the received signals are signals having a small amplitude of one volt or less and therefore cut off by the diode circuits 9. Using the foregoing circuitry, if preamplifiers are inserted into a maximum four channels, a two-dimensional array transmitting/receiving circuit having 8 channels in the slice direction can be realized.
In a two-dimensional array probe, when, for example, m transducers and n transducers are arranged in a scan direction and slice direction respectively, unless any measure is taken, the number of channels of a transmitting/receiving circuit increases with an increase in number of transducers. Consequently, a system having an extremely large-scale transmitting/receiving circuit becomes necessary. This is not practical. Moreover, even if the circuitry shown in FIG. 3 is employed, the number of cables is as many as four times larger than that attained when a conventional one-dimensional array probe is adopted. This poses a problem that the probe becomes heavy and hard to handle.